Tony Blair's former envoy to Baghdad yesterday launched his strongest attack on Britain and America, declaring that both countries had made catastrophic mistakes before and after the invasion of Iraq.

Abandoning his usual diplomatic language, Sir Jeremy Greenstock went much further than the prime minister, to say Britain had been "wrong" to claim that Saddam Hussein had stockpiles of chemical and biological weapons.

Speaking on BBC1's Breakfast with Frost, Sir Jeremy said: "There's no doubt that the stockpiles that we feared might be there are not there."

His remarks come as ministers brace themselves for Lord Butler's report on Wednesday into intelligence failures before the war. Peter Hain, the leader of the Commons, underlined ministerial unease yesterday, saying the report should not lead to a "witchhunt".

Sir Jeremy, who has said he had to work hard to persuade himself of the merits of the Iraq invasion while Britain's representative to the UN, said Mr Blair's decision to go to war was "understandable" because the intelligence about Iraq's banned weapons was "compelling".

"It's only, again with hindsight, when we saw that probably the Iraqis were cheating Saddam as well as misleading us, that the evidence is just not there. But the reason for doing this ... were actually quite compelling. We were wrong on the stockpiles, we were right about the intention."

Lord Butler, the former cabinet secretary, is expected to comment sharply on the way John Scarlett, the chairman of Whitehall's joint intelligence committee, interpreted secret information supplied by MI6.

Both the JIC and MI6 are expected to be attacked for failing to put raw intelligence reports into context, and notably the claim that Iraqi forces could deploy chemical and biological weapons within 45 minutes.

Sir Richard Dearlove, the head of MI6 - to be succeeded by Mr Scarlett at the end of the month - has said the 45-minute claim was given undue prominence in the government's dossier on Iraq's weapons programme published in September 2002. He has also said it should have been made clear that the claim referred to battlefield weapons and not long-range missiles.

Lord Butler has written to newspaper editors asking how they were briefed by government officials on the claim - highlighted by some parts of the media, notably the Sun and London's Evening Standard.

One of the main concerns of the Butler committee is the way raw intelligence from MI6 - some of it from defectors - was assessed by Mr Scarlett and his staff and the extent they were influenced by the political imperatives of Downing Street.

In a sign of Foreign Office distaste for the Iraq enterprise, Sir Jeremy criticised the US administration for accepting the "wrong analysis" from the discredited Iraqi exile Ahmad Chalabi, who played down the threat of insurgency.

Asked whether he was surprised by the postwar terrorist attacks, he said: "Some of us were, some of us weren't. There were different analyses and those making the decisions, which is Washington, chose the wrong analysis. They were influenced by Ahmad Chalabi ...

"There were papers in the American system, there were papers in the British system pointing out how difficult this was going to be."